Introduce proper types for packet_seq_t and stream_id_t.

Don't know where id_t was coming from, but this was certainly a bug.
Introduce mtu constant.

MTU is a fixed-size for ethernet right now but actually it should be a
per-link variable set up in each connection and sometimes probed using
separate MTU discovery mechanisms.
Log transmission events.
Make flags enum non-class so it can be used in bitops without casts.

Makes operating header flags easier.
Set internal header_len when setting up a requested header type.
Add stream write implementation.
Fix comment wording.
Add enqueue_stream implementation.